When firing semi-automatic and automatic firearms in law enforcement, military actions and in target shooting competition it is desirable to know when the magazine is about to run out of ammunition. Competition shooters need to know this information so they can be prepared to release the empty magazine and replace it with a full one with minimum loss of shooting time. In police and military operations the need to know is far more serious. Police officers and soldiers can lose their lives in the split second it takes to realize a magazine must be replaced, or an enemy or felon can escape in that time. And studies have shown that under the stress of a firefight it is nearly impossible for the user to keep accurate track of the cartridges fired and those remaining. Prior attempts to monitor the number of cartridges in a magazine by indicating the number expended or the number remaining have met with indifferent success, and by and large applied generally only to pistols and not other firearms. The apparatus often was large and cumbersome and required modifications to each firearm for retrofitting or required redesign for installation with original equipment manufacturers. Such systems did not distinguish between an empty condition and a jammed condition. There were often on-off switches: a real drawback when the user forgets to turn on the system. The prior art designs often relied solely on displaying a count of the number of cartridges spent or remaining, which meant that the shooter had constantly to be watching the display: there was no alarm that communicated urgency. And the brightness of the display was fixed and not adaptable to ambient light conditions or shooters with poorer eyesight. Often the display used an LCD which is not visible in dark or low light conditions, a serious problem in many law enforcement situations. Typically there was no way to test the battery or other power supply or the display to see that the monitoring system was working. The known systems also impeded the free-fall of a released magazine and were not waterproof, a serious shortcoming in police and military applications. And the addition of the monitoring system to the firearm often interfered seriously with the critical ergonomics of the firearm.